Launching space vehicles into space from locations of high frequency of lightning strikes, such as in Florida, presents a recurring problem and concern of the potential lightning induced damage to the space vehicles and payload systems prior to launch. This concern is especially acute for launches taking place during the high lightning season in the summer months. Those involved in the actual launch activities must account for the chaotic atmosphere particularly when a nearby lightning strike has been reported during the critical time path just before a launch. Currently, the assessment of the potential damage to the launch and space vehicle mainly depends on limited and primitive means. The entire launch vehicle including the payload, space vehicle, and an upper stage, if required, are subject to precautions offering modest protection against lightning, often without accurate knowledge of the threat. The decision to proceed with normal launch activities or to switch to retest and re-certification procedures is a difficult one which is subject to conjecture and often results in over-testing or under-testing. As more and more sensitive devices are used in the space vehicles and in order to achieve a balance between too much retest and too little retest, a more reliable lightning retest criterion to help make a real-time decision is needed. The purpose of such a criterion is, on the one hand, to avoid unnecessary and costly tests and delays, and on the other hand, to avoid launching damaged or degraded vehicles and payloads into space.
In order to effectively protect against lightning-induced electromagnetic effects, it is prudent to monitor and then protect selected lines and devices. There are two types of known lightning monitoring systems. One type of monitoring system is a remote off-line sensing system that measures the lightning current in the distant lightning channel. Another type of monitoring system is a proximal on-line monitoring system that measures direct parameters that stress the launch system and payload. One on-line monitoring system is the transient pulse monitoring system and has been used during Air Force launches. The proximal on-line monitoring system supports real-time launch operations. Ideally, the on-line monitoring system should be able to provide readings at as many sensor locations as needed. The initial placement of the sensors is guided by analyses for severe field points, that is, major entry points for lightning energies along critical and sensitive penetration paths. The present transient pulse monitoring system can only provide readings at six locations external to the space vehicle. High and low limits for sensors are used to start printed recorders or to alert the launch director based on external stress estimates that need to be refined to correlate with the internal stress at the circuit level. Flow-down analyses are needed to propagate the external stress to the internal circuits, followed by circuit coupling analysis and susceptibility analysis. Ideally, some internal circuit lines can be monitored to provide direct readings at the circuit level eliminating some of the simplifying assumptions and uncertainties in the mathematical models of the analyses. The present transient pulse monitoring system provides three types of stress readings including sheath current readings on the umbilical cable reading, electric field readings on selected surfaces, and magnetic field readings above the umbilical cable. Each type of reading presents a different mode of entry for the lightning energy coupled into the space vehicle system. These readings must be coordinated to give a combined indication of the stress at the circuit level.
The present Launch Range-operated cloud-to-ground lightning surveillance system is a remote sensing system. While it is valuable for general weather forecast purposes, it provides only indirect cloud-to-ground data of unspecified effects on the system circuits. To translate the distant environmental lightning data into system-specific stress data, many contractors at the present time rely on analyses that are quasi-static and back-of-the-envelope types of estimates for simple, now-obsolete launch configurations. Other existing monitoring systems are even more primitive having low fidelity and resulting in low confidence level information about lightning-induced transients that have resulted in many unnecessary retests and delays. Recent events have made many contractors aware of these limitations, and have moved them to use the proximal on-line monitoring system, and to re-examine their retest criteria.
The existing proximal on-line lightning monitoring systems use either analog or digital technology for detection and data processing. Currently two types of direct on-line lightning monitoring systems have been fielded. An analog type system, such as the transient pulse monitoring system, performs analog peak detection during continuous monitoring of lightning-induced transients. The digital monitoring system provides detailed waveform information of the transients. When deployed alone, the analog monitoring system gives no actual waveforms, while the digital monitoring system may miss significant events due to sampling limitations. The analog monitoring offers continuous front-end monitoring but lacks the actual waveform details that are needed and used by many contractors. It should be advantageous and cost-effective to combine these analog and digital systems into one integrated hybrid system that would avoid these shortfalls.
A correct determination of a Go and No-Go evaluation is vital to the launch operation that has obvious consequences on mission success. An efficient and effective retest algorithm is desired. The present-day algorithms used by contractors are based on the Boolean logic, indirect parameters, and inadequate measurements that often result in over-testing or under-testing. Over-testing leads to unnecessary delays, higher cost, and schedule impacts, while under-testing is dangerous because damage may not be discovered, resulting in the launch of a defective vehicle into space where no repair is feasible. Under-testing is most dangerous in that damage will not be discovered and damaged vehicles may be launched.
The miniaturization of electronic devices makes the launch system more sensitive and vulnerable to lightning-induced electromagnetic transients. Therefore, many systems have implemented procedures, often called lightning retest criteria or lightning damage search criteria for their launch processing operations following major lightning storms. Many existing lightning retest criteria and lightning damage search criteria are based on indirect parameters. This results in simplistic, misleading and erroneous retest decisions. During the critical moments shortly before launch, little time is allowed for detailed and elaborate analysis of the situation that may arise due to lightning events. However, many complicated analyses are generic processes that can be performed well in advance of a launch, and the analytical results with appropriate system configurations can be incorporated into the lightning effects evaluation algorithm in the form of lookup data tables.
Instead of waiting for nature to deliver all types of lightning over a long period of time, parametric studies using mathematical models can be carried out to analyze many possible scenarios. Three kinds of analyses are needed to support the formulation of a dependable lightning retest evaluation method. The three kinds of analyses are electromagnetic (EM) field coupling analysis, circuit coupling analysis, and susceptibility analysis. These analyses can be performed during pre-launch periods.
Field coupling analysis is an EM field coupling analysis that is based on the scattering theory in terms of Maxwell equations and is used to characterize the induced electromagnetic transients on the segments in the model space due to lightning-generated electromagnetic fields. Because of the complexity of the structures and geometry, a method of moments computer code or a finite difference computer code is needed to S incorporate essential geometry and physics into the analysis. Analysis results provide the stress drivers from any selected location for the stress evaluation of identified circuits and devices in a later circuit analysis. Each structure is segmented to represent locations where the responses can be monitored.
Circuit analysis is a coupling analysis that determines the most likely entry point of lightning energy. Critical circuits or devices for a particular space system must be first identified. The stresses induced by lightning on the circuits and devices in the system will be evaluated. Circuit models for these circuits will be built for a circuit analysis code such as the industry standard code named SPICE using the location drivers from the EM field coupling analysis. The SPICE code evolved from the research project at UC Berkeley and is available commercially in many versions. The SPICE code remains a powerful circuit analysis tool for electrical circuits. The outputs of the circuit analysis are stress energies coupled into the circuits. Stress energy ratios for different location drivers provide initial individual sensor channel weights. The weights with drivers of various severity will also yield a stress profile as a stress curve in energy versus a sensor reading for a particular circuit at a given location.
Susceptibility analysis of the space system will determine the susceptibility of the system to energy stresses. A Wunch-Bell type of damage mechanism is usually used in the damage assessment. The Wunch-Bell model for damage and its applications can be found in the public literature and is well known by those skilled in the art of lightning testing. This susceptibility analysis will yield threshold energies that cause damage to the circuits. The susceptibility analysis aids in the identification of sensitive circuits or devices in the system. Electric field sensor readings inside known layers of enclosures can be compared to a Fourier transform, in the time domain, of an analytical Electric field prediction for determining a certain threat level. Other points can also be found for less severe threat levels to obtain the shape for the Electric sensor stress curve. Without performing the susceptibility analysis for the circuits, this stress curve only defines a generic shape for an allowable electric field stress level. This generic stress curve lacks the details and the characteristics of the selected circuits and, therefore, the susceptibility analysis should be circuit specific for generating suitable strength levels. The threshold energy for the most sensitive circuits or devices will provide a strength line. Certain safety margins, e.g. 6 dB, are required for many electromagnetic compatibility considerations and can be built-in by moving the strength line downwards on the retest plot thereby providing safety margins desired for the decision-making process.
As micro-technology advances, miniature probes and sensors may become available to allow closer placement of the probes and sensors to circuits and devices of interest reducing the need for circuit analyses. Because there is no a priori knowledge about the next lightning event and the general topology of launch platforms, it is necessary to deploy multiple sensors to cover all possible entry points of lightning energies. Multiple sensor deployment increases confidence levels of detection by confirming significant events and by discarding spurious events through correlated multiple sensor inputs.
A lightning retest criterion should include all the important parameters that affect the system under lightning monitoring. The first parameter is the stress induced by lightning on the system, and the second parameter is the intrinsic strength of the system to withstand the stress without suffering damage. Lightning, by virtue of transient electromagnetic field scattering from the system, will first induce electromagnetic stresses on the external surface of the system such as the payload faring and umbilical cable sheath. The external stress, in terms of induced electromagnetic fields or currents on the surface of the system will then propagate through available paths to the circuits and devices in the interior, and discharge undesirable energy there causing damage. A validated transient pulse monitoring system as installed for a particular payload will give the actual external stress, and analyses will indicate the effect of the external stress over a wide range of lightning threats. The stress at circuit levels is the true reason for damage concerns, and stress is stated in terms of stress energy and voltage waveforms by circuit coupling analyses.
However, stress alone will not indicate whether or not the circuit will be damaged. The strength, that is, the tolerance, of the circuit is determined by the susceptibility analysis. A comparison of the stress and strength in the susceptibility analysis yields an indication of how well the circuit will tolerate a given stress. A retest criterion can then be formulated with desired safety margins. Even if the stress is very severe, a circuit that has strength to withstand that stress should not be damaged. The strength of a device or circuit is an intrinsic characteristic of such a device or circuit. A good example of the strength of a device or circuit is the burn-out damage threshold. Abundant experimental data is available for various types of devices such as diodes, transistors, digital circuits, and other basic electronic components. This data has been fitted to the Wunch-Bell-Jenkins-Durgin empirical failure models. A conservative sample burn-out damage energy threshold is one micro-joule for many popular sensitive electronic devices.
The historical account has shown that the present monitoring systems are inadequate in detecting lightning-induced transients, and the retest criteria produce decisions subject to conjecture and results in over-testing or under-testing. As more and more sensitive devices are used in launch and space vehicles and other electronic systems, a more reliable lightning effects monitoring and retest evaluation method is needed. The inadequacies and other disadvantages of the present monitoring systems can be solved or reduced using the present invention.